It is desirable to provide protection for aircraft fuel tanks, which contain fuel and volatile gasses. Fuel tanks can be protected by providing inert gas to the fuel tank ullage, to reduce the oxygen content, and thus combustibility of the tank components. Ullage is the space within the fuel tank which is unfilled by fuel. The goal of these methods is to reduce oxygen content to below a threshold of flammability. For military operations, this threshold is generally 9%, while for commercial operations, this threshold is generally 12%. Maintaining oxygen below the threshold level ensures that combustion is not possible within the tank. Supplying inert gas to fuel tank ullage will be referred to herein as “inerting fuel tank.”
A variety of inert gasses and methods of inerting fuel tanks are known. Inert gasses which have been previously considered, or which are currently in use include nitrogen, nitrogen enriched air, carbon dioxide, exhaust gas and flame suppressing agents such as Halon 1301.
Prior methods of inerting a fuel tank typically utilize air with a high nitrogen content as the inert gas. In a first prior method, stored liquid nitrogen, which can be converted to a gaseous form, is supplied to a fuel tank as needed or desired. In a second prior method, compressed atmospheric air drawn from a main engine is fed through diffusion elements within a diffusion system, which filter oxygen out of the atmospheric air. The filtered air, which is nitrogen enriched and oxygen depleted and thus sufficiently inert, is compressed and cooled, and utilized to inert the fuel tank.
In the second prior method, compressed atmospheric air may be drawn as“bleed air” directly from a main engine. Bleed air is typically drawn from within the engine, where the engine turbines have already compressed incoming air. This is a convenient source of compressed air. However, removal of bleed air from the engine reduces engine efficiency. Thus, it would be desirable to provide a system that does not use bleed air.
Further, the diffusion elements which filter oxygen out of the compressed air have a maximum flow capacity. Thus, bleed air systems which enrich atmospheric air through the use of nitrogen diffusion elements have a maximum capacity for providing inert gas to fuel tanks. This maximum capacity means that such filtering systems may not be able to provide sufficient inert gas during all flight phases. For example, during a quick descent, oxygen may flow into the fuel tanks faster than it can be replaced by the inert gas system.
The capacity of a bleed air filtering system may be increased by increasing the number of diffusion elements. This allows inert gas throughput capacity to increase linearly with the number of diffusion elements. However, this adds bulk and weight to the aircraft. Therefore, it would be desirable to provide a system which has a widely varying throughput capacity.
Efforts have been made to utilize hybrid systems combining bleed air powered systems with systems utilizing stored liquid nitrogen. However, such hybrid systems may require more space than either system individually and thus may increase the weight of the aircraft and decrease the amount of space available for other purposes. Further, servicing systems, such as systems for refilling liquid nitrogen tanks, are not generally available at airports.
A system is thus needed having a widely variable capacity to provide inert gas to fuel tanks, and which does not decrease engine efficiency by utilizing bleed air.